1. Field of the Invention
The present invention relates to a thickness or gage control system for a rolling mill and in particular to a gage or thickness control system for eliminating the influence of roll eccentricity.
2. Description of the Prior Art
In these years, there has been an increasing demand for accuracy in the thickness of rolled strips and remarkable progress has been made in gagemeter type automatic thickness control systems on the basis of the so-called BISRA-AGC (automatic gage control developed by BRITISH IRON AND STEEL RESEARCH ASSOCIATION).
This gagemeter type automatic thickness control systen controls values such as a thickness command hd, no-load roll gap S, roling pressure P, and mill modulus Km so as to satisfy the following equation: EQU hd - (S + P/Km) = 0
These values are indispensible for controlling the thickness of strip in a rolling mill.
However, the hitherto known gagemeter type automatic thickness control system has suffered from disadvantages in that the presence of eccentricity in the respective rolls leads to failure in maintaining the roll gap constant, which makes the purpose of the thickness control itself utterly meaningless. In other words, the hitherto known gagemeter type control system is so designed as to decrease the roll gaps when the rolling pressure is increased on the assumption that the an increase in the rolling pressure has been caused by the increase in the thickness of a strip on the input side. However, when the roll gap is decreased due to roll eccentricity, the rolling pressure will be correspondingly increased, so that the control system will function so as to decrease the roll gap notwithstanding the need to increase the gap. Accordingly, it is an important problem imposed on the gagemeter type automatic gage control system to remove or exclude the influence of roll eccentricity.
Meanwhile, many attempt have so far been made to overcome the above problem. However, most of these attempts have failed to meet an intended success, because of over complicated construction or the failure to attain the desired accuracy, with the result of the necessity to resort to the skill or experience of operator to solve the above problem. For example, as one of the simplest attempt among the above referred to attempts, it has been proposed to modify the automatic gage control system such that the rolling pressure control is incorporated with a feed back loop including a resonance type filter for passing only the components of roll eccentricity frequency fe thereby to control the rolling pressure so as to cancel the roll eccentricity component in variation of the rolling pressure. However, this attempt has encountered disadvantages in that the so-called resonance type filter designed to have a resonance frequency at the roll eccentricity frequency fe have usually a resonance bandwith (sensitive zone width) of frequencies broader than the intended bandwith, whereby the signals having frequencies close to that of the signal to be passed therethrough, furthermore, exact tuning is very difficult too.
It has also been proposed as another attempt that the feed back quantity is subjected to Fourier analysis to extract the roll eccentricity component, which is then utilized as a command quantity for the thickness control. In this case, however, an adjustment of gain and correction of phase delay in the servo system has to be made with high accuracy, since the command is given as the eccentricity correction signal. This in turn requires a very complicated adjustment with the aid of an expensive computer. Besides, it involves a risk in that the roll eccentricity will be quite undesirably amplified, if the adjustment of gain and phase is erroneously effected.